1. Field of the Invention
The present invention relates generally to a display device, and particularly, to a liquid crystal display device capable of suitably displaying video (an image) irrespective of the total number of dots in a horizontal period of an input video signal.
The present invention further relates to a pixel corresponding display device for displaying video in units of pixels, and particularly, to a pixel corresponding display device for suitably displaying video by always keeping a phase relationship between a clock signal in a case where an inputted video signal is subjected to analog-to-digital conversion (hereinafter referred to as A/D conversion) and pixels composing a video signal in a stable state.
2. Description of the Prior Art
[1] In a liquid crystal display device, one of dots represented by dot data of an input video signal and one of pixels composing a liquid crystal panel are synchronized with each other in one horizontal scanning period, to display video. Line data representing one horizontal scan line out of an arbitrary number of line data in one vertical scanning period of the input video signal is displayed in correspondence with one line in the vertical direction of the liquid crystal panel. The line data is a collection of pixel data.
In recent years, computers with a large variety of specifications have been fabricated. A video a signal shown in FIG. 1a and a video signal shown in FIG. 1b differ in the total number of dots in a horizontal period (hereinafter referred to as the total of horizontal dots), for example, even if they are XGA (Extended Graphic Array) video signals outputted from various types of computers. In the XGA video signals, the respective total numbers of dots within a video effective period in the horizontal period (hereinafter referred to as the number of horizontal effective dots) are common. That is, the number of horizontal effective dots of the XGA video signal is 1024.
Sampling clocks for sampling 1024 dots within the video effective period in the horizontal period of the inputted XGA video signal are generated on the basis of a horizontal synchronizing signal of the inputted XGA video signal. Consequently, a method of generating the sampling clocks must be changed depending on the total of horizontal dots of the inputted XGA video signal. Therefore, it is necessary to recognize the total of horizontal dots of the inputted XGA video signal in order to generate the sampling clocks.
Conventionally, a table storing the total of horizontal dots has been prepared for each of types of XGA video signals, the type of the XGA video signal is judged from the characteristics of the XGA video signal inputted from the computer, and the total of horizontal dots corresponding to the judged type is selected from the table, thereby recognizing the total of horizontal dots of the inputted XGA video signal. However, this method cannot cope with the XGA video signal generated by a computer with a new specification.
Therefore, the applicant of the present invention has developed a method of generating sampling clocks suitable for a plurality of types of video signals whose number of horizontal effective dots has been known and which differ in the total of horizontal dots, to file an application (see JP-A-11-311967).
Specifically, a horizontal video start position HS (see FIGS. 1a and 1b) and a horizontal video end position HE (see FIGS. 1a and 1b) are detected for each of horizontal lines, to calculate the number of sampling clocks corresponding to the distance from a horizontal video start position and a horizontal video end position of an input video signal on the basis of the horizontal video start position and the horizontal video end position which have been detected.
When the number of sampling clocks corresponding to the distance between the horizontal video start position and the horizontal video end position of the input video signal is smaller than the known number of horizontal effective dots “1024”, the frequency of the sampling clocks is controlled in such a direction as to be increased. When the number of sampling clocks corresponding to the distance between the horizontal video start position and the horizontal video end position of the input video signal is larger than “1025” which is larger by one than the known number of horizontal effective dots, the frequency of the sampling clocks is controlled in such a direction as to be decreased.
A When the number of sampling clocks corresponding to the distance between the horizontal video start position and the horizontal video end position of the input video signal coincides with 1024 or 1025, the phase of the clocks is delayed by a value corresponding to at least one clock in several nano units.
The horizontal video start position and the horizontal video end position of the input video signal are detected by comparing the input video data after the AD conversion by the A/D converter with a predetermined threshold value. However, the position detection precision is greatly affected depending on how an analog input video signal inputted to the A/D converter is dull.
Threshold values used for detecting the horizontal video start position and the horizontal video end position of the input video signal have been conversely fixed, and are forced to be determined so as to be adapted to an input video signal having a low luminance out of input video signals. Accordingly, the dullness of the analog input video signal inputted to the A/D converter greatly affects the position detection precision.
Since the dullness significantly appears at the falling edge of the analog input video signal, the detection precision of the horizontal video end position is particularly decreased in the conventional example.
[2] In a liquid crystal display which is one type of pixel corresponding display device, one of pixels represented by pixel data of a video signal and one of pixels composing a liquid crystal panel are synchronized with each other in one horizontal scanning period, to display video. Line data representing one horizontal scan line (hereinafter referred to as a line) out of an arbitrary number of line data in one vertical scanning period of the video signal is displayed in correspondence with one line in the vertical direction of the liquid crystal panel. The line data is a collection of pixel data.
Data handled inside by a computer is a digital signal, and the video signal is also generated as a digital signal in units of pixels. However, a CRT display which has been conventionally used as a display device is an analog device. Accordingly, the video data generated in the computer is converted into an analog video signal in the computer, to output the analog video signal to the CRT display.
On the other hand, the liquid crystal display is a digital device. Accordingly, the video signal fed as an analog signal from the computer must be subjected to A/D conversion. Therefore, sampling clocks for performing the A/D conversion must be reproduced on the side of the display. Conventionally, the sampling clocks for performing A/D conversion have been reproduced on the basis of only a horizontal synchronizing signal. However, it is not ensured that the phase relationship between the horizontal synchronizing signal and pixels composing the analog video signal is always kept in a correct state, and the total number of clocks within one horizontal period is unclear. Consequently, a system for adjusting the frequency and the phase of the clocks is required.
The following is a method of automatically adjusting the frequency and the phase of the sampling clocks in response to an input signal. The total of sampling clocks in one horizontal period is adjusted to adjust the frequency of the sampling clocks such that the number of horizontal effective pixels composing input video reaches a predetermined value. A horizontal synchronizing signal of an input video signal is fed to a sampling clock generation circuit through a delay circuit, and the amount of delay in the delay circuit is adjusted, to adjust the phase of the sampling clocks.
However, the input signal must satisfy the following conditions in order to accurately adjust the frequency and the phase of the sampling clocks in this method.
Condition 1; In order to accurately detect the number of horizontal effective pixels, at least one horizontal effective start point whose level exceeds a threshold level for horizontal effective start point judgment and at least one horizontal effective end point whose level exceeds a threshold level for horizontal effective end point judgment exist within one field period in the input video.
Condition 2; the first condition is satisfied continuously for a plurality of field periods.
In the case of almost all screen saver images, for example, an object is to prevent a CRT from being seized. Accordingly, a frame whose position is fixed is not displayed, a background image is taken as an image whose level is close to a black level, and an image to be displayed is further displayed as a moving image. When the screen saver image is inputted, therefore, the number of horizontal effective pixels cannot be accurately detected.
Furthermore, a horizontal RAMP image has no sharp edge for specifying a horizontal video start position or a horizontal video end position, and is easily affected by a noise component or a horizontal jitter component. When the horizontal RAMP image is inputted, therefore, the number of horizontal effective pixels cannot be accurately detected.
That is, it has been conventionally impossible to distinguish a special video signal and a normal video signal, described above. Accordingly, accurate clock automatic adjustment cannot be performed.